1. Field of the Invention
The present invention relates to the method for producing citric acid on a continuous basis, after fixing Aspergillus niger B60 as mildews, in a dual hollow fiber membrane bioreactor.
2. Description of the Related Art
Recently, citric acid is commonly produced by fermentation, 20 percent of them is produced by surface culture and remaining 80 percent by submerged culture (Sodeck et al., Proc. Biochem. Oct./Nov., 9, 1981). However, such methods according to the prior art are batch operation which have a disadvantage of lower productivity in comparison with continuous operation according to present invention. As the existing continuous methods improved on the batch method, there are (1) a method utilizing a fermenter equipped with an agitator (Kristiansen and Sinclair, Biotechnol. Bioeng., 21, 297, 1979), (2) a method utilizing air-lift reactor after fixing Aspergillus niger mycelium to alginate bead (Vaija et al., Appl. Biochem. Biotechnol., 7, 51, 1982) and (3) a method utilizing tower reactor after the fixation of Aspergillus niger to polyacrylamide (Horitsu et al., Appl. Microbiol. Biotechnol., 22, 8, 1985).
The continuous operations in the existing method (1).about.(3) brought higher productivity, but less concentration than the batch operation as in the case of other continuous operations, and required higher costs for the process of purification.
Moreover, when carriers are used to fix mildews, there was a tendency to weaken the carriers due to an excessive growth of cells. Leaving the carriers, cells, also, grow in liquid culture medium to cause many operational difficulties in the case of continuous operation.